CCD and CMOS image sensors used in imaging devices such as a video camera, a digital camera, and the like convert a quantity of brightness and darkness of light incident as an external signal to a charge quantity, i.e., perform what is called photoelectric conversion, and sequentially process the electric signal to thereby generate a digital image. In such CCD and CMOS image sensors, since distortion resulting from sampling is generated in an image having a spatial frequency smaller than a pixel pitch of incident light and a pattern (moiré) different from an original image occurs, the CCD and CMOS image sensors are constituted to include an optical low pass filter in order to prevent the occurrence of the moiré. A specific optical low pass filter has a function of cutting the vicinity of a frequency (sampling frequency) of the pixel pitch which enters the image sensor by slightly separating an incident two-dimensional image in horizontal and vertical directions, and is so devised as to prevent the occurrence of the moiré phenomenon by the function.
FIGS. 11A to 11C are schematic views specifically showing an example of a configuration of a low pass filter 100 which is disposed at a position before light becomes incident on the image sensor, and separates the light into four light beams, and a polarization state of the light. FIG. 11A is a three-dimensional schematic view of the low pass filter 100 in which a first optical path separation birefringent plate 101, a wave plate 102, and a second optical path separation birefringent plate 103 are disposed in the order of incidence of the light. In addition, FIG. 11B is a plan schematic view showing a position of light (light axis) having passed through the first optical path separation birefringent plate 101 and a polarization state thereof, while FIG. 11C is a plan schematic view showing a position of light (light axis) having passed through the second optical path separation birefringent plate 103 and the polarization state thereof.
A description is given of a state of light in the process where incident light passes through the low pass filter 100. In FIG. 11A, it is assumed that light incident on the low pass filter 100 is in a random polarization state, and travels in parallel with a Z-axis direction. Herein, the randomly polarized light is considered by dividing the randomly polarized light into two polarization components, and a component in parallel with an X-axis direction (hereinafter referred to as an A-polarized light) and a component in parallel with a Y-axis direction (hereinafter referred to as a B-polarized light) are considered. When the light becomes incident on the first optical path separation birefringent plate 101, the A-polarized light passes therethrough in a straight-ahead direction with respect to the incident direction of the light, while the B-polarized light follows a course separated from that of the A-polarized light to pass therethrough. It is to be noted that positions where the A-polarized light and the B-polarized light pass through the first optical path separation birefringent plate 101 are denoted by reference numerals 101a and 101b respectively, and these position are indicated in a transmission surface (X-Y plane) of the first optical path birefringent plate 101, as shown in FIG. 11B.
The light separated into an optical path 104a of the A-polarized light and an optical path 105a of the B-polarized light subsequently becomes incident on the wave plate 102. The wave plate 102 has a function of performing phase modulation on the light incident in a specific vibration direction such as the A-polarized light and the B-polarized light such that the A-polarized light (component) and the B-polarized light (component) have the same light quantity. The light having passed through the wave plate 102 in this manner travels straight with the A-polarized light and the B-polarized light mixed therein, and the light in correspondence to the optical path 104a of the A-polarized light is assumed to pass through the wave plate 102 and become incident on the second optical path separation birefringent plate 103 as the light of an optical path 104b, while the light in correspondence to the optical path 105a of the B-polarized light is assumed to pass through the wave plate 102 and become incident on the second optical path separation birefringent plate 103 as the light of an optical path 105b. At this point, in the second optical path separation birefringent plate 103, the A-polarized light and the B-polarized light are separated such that the A-polarized light and B-polarized light follow different courses in the same manner as in the above-described first optical path separation birefringent plate 101. At this point, the light separation direction in the first optical path separation birefringent plate 101 and the light separation direction in the second optical path separation birefringent plate 103 are made to be orthogonal to each other.
The reason why the separation directions are made to be orthogonal to each other as described above is that, since pixels of the image sensor are two-dimensionally arranged, in order to prevent the moiré with respect to two orthogonal directions of the arrangement, the light is separated in the separation directions matching with the arrangement directions of the pixels. In addition, the width of the separation (separation distance) differs according to the pitch of the pixels and the spatial frequency to be cut. Further, when a pixel shape is square, it is effective to have the same separation distance in the X direction and in the Y direction. However, in the case of the image sensor using, e.g., rectangular pixels (length of one pixel in Y direction>length of one pixel in X direction), a high priority is given to the prevention of the moiré in the X direction. Consequently, the separation distances in the X and Y directions may be different, and a quadrangle obtained by joining four separated points is not limited to a square and the quadrangle may be a parallelogram. Further, as described above, when a high priority is given to the prevention of the moiré in the X direction, there is a case where two-point separation only in the X direction is sufficient.
From each of the optical paths 104b and 105b of the incident light, the A-polarized light and the B-polarized light are separated in the second optical path separation birefringent plate, and pass therethrough. At this point, positions where the A-polarized light and the B-polarized light separated from the optical path 104b pass through the second optical path separation birefringent plate 103 are denoted by reference numerals 103a and 103b respectively, and positions where the A-polarized light and the B-polarized light separated from the optical path 105b pass through the second optical path separation birefringent plate 103 are denoted by reference numerals 103c and 103d respectively. Further, these positions are indicated in a transmission surface (X-Y plane) of the second optical path separation birefringent plate 103, as shown in FIG. 11C. Thus, when the light having components of both of the A-polarized light and the B-polarized light becomes incident, two orthogonal light components pass through the second optical path separation birefringent plate at two different positions for each light component.
There is reported, as the wave plate 102, a wave plate having, e.g., a function of converting incident A-polarized light and B-polarized light to circularly polarized light with an ellipticity close to 1 by using crystal as a ¼ wavelength plate (hereinafter referred to as a λ/4 plate) such that each of the incident A-polarized light and B-polarized light has the equivalent light quantity (Patent Document 1).